In managing livestock animals using present methods, visible characteristics or phenotypic traits are used to predict how an animal will grow, and thus how the animal should be fed to most profitably achieve market condition. The object of a livestock industry is to convert feed into meat, and much is known about growth patterns of livestock.
Fat deposition, or the amount of intramuscular fat and back fat on an animal carcass, is important to industry participants because carcasses exhibiting desired amounts and proportions of such fats can often be sold for higher prices than carcasses that exhibit divergences from such desired amounts and proportions. Further, the desired carcass fat deposition often varies among different markets and buyers, and also often varies with time in single markets and among particular buyers in response to public demand trends with respect to desired of fat and marbling in meat.
Weight gain by a livestock animal during its growth and development typically follows a tri-phasic pattern that is carefully managed by commercial producers and finishers. The efficiency of dietary caloric (feed) conversion to weight gain during an increment of time varies during three growth phases; a first phase of growth comprises that portion of a livestock animal's life from birth to weaning, and is not paid much heed by commercial feeding and finishing operators.
A second growth phase comprises that portion of a livestock animal's life from weaning to attainment of musculoskeletal maturity. Feed conversation efficiency is relatively high during this phase; livestock producers usually restrict caloric intake, which has the effect of causing this phase to be prolonged but also typically results in animals with larger frames, which is the aim of dietary management during this phase. During the second growth phase weight gain is associated with skeletal mass and muscle mass accumulation primarily.
During a third growth phase, after an animal has attained musculoskeletal maturity, the efficiency of feed conversion is reduced, such that it requires more feed to increase an animal's weight. Thus, during the second phase of growth, a typical steer could convert 5 to 6 pounds of feed into one pound of weight gain. Upon entering the third phase, feed conversion efficiency typically decreases, such that 7 up to 10 or more pounds of feed are required to produce one pound of gain.
During the third phase livestock feeders significantly increase the caloric content of animals' rations. During the third growth phase weight gain is associated with fat accumulation primarily. In a steer weighing 900 pounds at the end of the second phase, of that 900 pounds, typically 350 pounds will be red meat. At the end of the third phase, the steer would typically weigh 1400 pounds and typically 430 pounds will be red meat.
In the cattle industry, initially a cow/calf operator will breed bulls to cows, birth calves from the cows, and allow the calves to feed on their mother's milk until they are weaned some months after birth. This is the first phase of growth of the calf.
After weaning, the calf enters the second stage of growth where it is fed to grow to its full skeletal size. This commonly called the “backgrounding” phase during which musculoskeletal maturity is achieved. When the animal has reached its full size, it enters the third phase of growth where the fully grown animal puts on weight.
Typically it is at the start of the third stage of growth that the animal enters a finishing feed lot. In the feed lot the object is to feed the animal the proper ration so that it will most quickly obtain the proper market characteristics that are desired at that given time. At present, for instance it is desirable to have beef that is well marbled, i.e., it has considerable intramuscular fat in the meat. At other times it may be desirable to have lean meat with very little intramuscular fat. The price the feed lot owner attains for cattle when sold to the packer can vary significantly depending on marbling of the meat.
Presently, cattle entering a feed lot are divided into groups according to phenotypic characteristics such as estimated age, frame size, breed, weight and so forth. By doing this the feed lot owner is attempting to group the cattle so that the group can be penned together and fed the same ration and will be ready for market at the same time.
The phenotype of an animal is the visible characteristics of the animal, which results from the interaction between the animal's genetic makeup and its environment. Conventional management techniques group cattle according to uniform phenotypic traits and then keep the environment constant for each animal in the group in hopes that the group will together achieve a different phenotype at some later date. The genetic makeup of any individual steer is a significant factor in the ability of that individual steer to grow in the same manner as another steer of the same phenotype.
Considerable variation in phenotypes is observed at the end of the third phase among cattle that entered the third phase with a substantially uniform phenotype, despite having been subjected to the same environmental factors as with conventional management methods. It is not uncommon for a pen of cattle, each having a weight within a range of 100 pounds going into a feeding pen, to have weights varying in a range of 300 pounds or more coming out of the pen for slaughter. It is also known that the feed conversion rate of cattle varies to some degree. Since feed represents a major cost to the feed-lot operator, it is more profitable to feed those cattle with a higher feed conversion rate, since an animal that converts a ton of feed into 200 pounds of saleable body weight is more profitable than an animal that converts the same ton of feed into only 180 pounds of saleable meat.
The timing of slaughter is based on the mean visible condition of the group of cattle in each pen, resulting in a wide variation in carcass weight and ensuring that grading premiums for carcasses of a desired condition of weight and fat are not met for a significant number of cattle. In a typical pen, a number of the cattle in the pen would have been at the desired carcass condition earlier, but by the time they are slaughtered they are over fat. Similarly, many cattle could readily achieve the desired carcass condition if fed longer. However, conventional management techniques require that all the cattle in the pen are slaughtered at the same time.
A dairy cattle operator is faced with similar issues as packers, feeders and cow/calf operators. Dairy cattle are also segregated into groups based upon phenotypic traits even though genotype can affect milk production. In particular, the time period from calving through to peak lactations is the most stressful period in the life of the dairy cow. During this time, the animal usually falls into negative energy balance because the daily feed intake, although increased, is unable to keep pace with the increased energy demand of lactation. Since certain genotypes affect energy balance, management of animals by genotype will be important for efficient dairy production. Furthermore, the animals' genetic predisposition to lay down fat also impacts milk production.
It is well known to those skilled in the art that single nucleotide polymorphisms (SNPs) can provide a useful way in which to distinguish different alleles of a gene. Furthermore, when the presence of a SNP can be associated with a specific phenotype, the SNP operates as a powerful marker and can be used to predict phenotypic outcomes based on an animal's genotypic makeup. By identifying animals with a particular genotype, with respect to SNP alleles, it is possible to identify or select for animals that display desirable phenotypes, as compared to animals lacking the desired genotype.
In the beef cattle industry in many countries, animals that gain weight better than their counterparts are more desirable because they finish faster. Producers receive the most money when the animal has an optimal layer of fat, and are penalized for overly thin or overly fat animals. As a result of this, genes that influence the appetite pathway are of particular interest to the industry. Identification and characterization of the genes involved in appetite and metabolism aids producers in the selection of the best possible animals that will produce the most monetary gains.
The gene for Pro-Melanin-Concentrating Hormone (PMCH) encodes three neuropeptides: Neuropeptide-Glycine-Glutamic Acid (NGE), Neuropeptide-Glutamic Acid-Isoleucine (NEI), and Melanin-Concentrating Hormone (MCH) (Nahon et al. 1989). MCH is the most extensively studied product of PMCH. MCH has been shown to stimulate feed intake, as well as to modulate metabolic function in rodents (reviewed by Pissios et al. 2006). MCH is predominantly expressed in the mammalian hypothalamus and has also been shown to be expressed in other tissues including the thyroid, spleen, intestine, and testes of mice and rats (Baker 1994). The role of MCH in the appetite pathway has been supported by findings that MCH mRNA is up-regulated in fasted mice (Shimada et al. 1998) and humans (Gavrilla et al. 2005), and that MCH knock-out mice are lean, exhibiting decreased appetite and increased metabolic rate (Shimada et al. 1998). Conversely, over expression of MCH has been shown to lead to obesity and insulin resistance, mediated by increased appetite and reduced metabolism (Ito et al. 2003; Ludwig et al. 2001).
PMCH has been mapped in a number of mammalian species. In humans PMCH is located on chromosome 12q23-q24 (Pedeutour et al. 1994), on mouse chromosome 10 (Kapfhamer & Burmeister 1994), and on cattle chromosome 5 (Stone et al. 2002). Evidence that PMCH may play a role in fat production in cattle comes from the findings that quantitative trait loci (QTL) for backfat exist in the same region of BTA5 (Casas et al. 2000; Li et al. 2004). Thus, the identification of polymorphisms in PMCH that are causal or that are linked to important carcass traits in cattle could allow the implementation of accurate and inexpensive genetic assays to identify or select for animals that display a desirable phenotype.